A deformable membrane assembly comprises a deformable membrane (70) in contact with a body of fluid (90) and a selectively operable adjuster for adjusting the pressure of the fluid (90) for causing distension of the membrane (70) in accordance with a predefined form. The membrane is held peripherally by a bendable supporting member (50) that is coupled to a fixed support by at least three engaging members (82) at spaced locations round the supporting member (50). An engaging member (82) is provided at or proximate at least one point round the supporting member (50) where the profile of the supporting member (50) corresponding to the predefined form of the membrane (70) exhibits locally maximum or minimum curvature in the direction of distension of the membrane (70).